SOFC with cathode exhaust partial bypass of the ATO and additional air cooling of a hotbox

ABSTRACT

One method of operating a fuel cell system including splitting a cathode exhaust from one or more fuel cell stacks in the system into a majority cathode exhaust stream comprising more than 50% of the cathode exhaust and a first cathode exhaust bypass stream, providing the majority cathode exhaust stream to an inlet of an anode tail gas oxidizer (ATO) containing a catalyst and providing the first cathode bypass stream downstream of the catalyst such that it bypasses the catalyst. Another method includes providing an air inlet stream to the SOFC system via a main air inlet, providing the air inlet stream from the main air inlet to a cathode recuperator, and providing a cooling medium to a heat exchanger to cool the cathode recuperator.

FIELD

The present invention is directed to fuel cell systems, specifically tocomponents for a solid oxide fuel cell (SOFC) system hot box.

BACKGROUND

Fuel cells, such as solid oxide fuel cells, are electrochemical deviceswhich can convert energy stored in fuels to electrical energy with highefficiencies. High temperature fuel cells include solid oxide and moltencarbonate fuel cells. These fuel cells may operate using hydrogen and/orhydrocarbon fuels. There are classes of fuel cells, such as the solidoxide regenerative fuel cells, that also allow reversed operation, suchthat oxidized fuel can be reduced back to unoxidized fuel usingelectrical energy as an input.

FIGS. 1-9 illustrate a prior art fuel cell system described in U.S.Published Application 2010/0009221 published on Jan. 14, 2010 (filed asSer. No. 12/458,171 and incorporated herein by reference in itsentirety. Specifically, with reference to FIGS. 1, 2A, 2B and 3A, anintegrated fuel cell unit 10 is shown in form of an integrated solidoxide fuel cell (“SOFC”)/fuel processor 10 having a generallycylindrical construction. The unit 10 includes an annular array 12 ofeight (8) fuel cell stacks 14 surrounding a central axis 16, with eachof the fuel cell stacks 14 having a stacking direction extended parallelto the central axis 16, with each of the stacks having a face 17 thatfaces radially outward and a face 18 that faces radially inward. As bestseen in FIG. 3A the fuel cell stacks 14 are spaced angularly from eachother and arranged to form a ring-shaped structure about the axis 16.Because there are eight of the fuel cell stacks 14, the annular array 12could also be characterized as forming an octagon-shaped structure aboutthe axis 16. While eight of the fuel cell stacks 14 have been shown, itshould be understood that the invention contemplates an annular array 12that may include more than or less than eight fuel cell stacks.

With reference to FIG. 1, the unit 10 further includes an annularcathode recuperator 20 located radially outboard from the array 12 offuel stacks 14, an annular anode recuperator 22 located radially inboardfrom the annular array 12, a reformer 24 also located radially inboardof the annular array 12, and an annular anode exhaust cooler/cathodepreheater 26, all integrated within a single housing structure 28. Thehousing structure 28 includes an anode feed port 30, an anode exhaustport 32, a cathode feed port 34, a cathode exhaust port 36, and an anodecombustion gas inlet port 37. An anode exhaust combustor (typically inthe form an anode tail gas oxidizer (ATO) combustor), shownschematically at 38, is a component separate from the integrated unit 10and receives an anode exhaust flow 39 from the port 32 to produce ananode combustion gas flow 40 that is delivered to the anode combustiongas inlet 37. During startup, the combustor 38 also receives a fuel flow(typically natural gas), shown schematically by arrow 41. Additionally,some of the anode exhaust flow may be recycled to the anode feed port30, as shown by arrows 42. In this regard, a suitable valve 43 may beprovided to selectively control the routing of the anode exhaust flow toeither the combustor 38 or the anode feed port 30. Furthermore, althoughnot shown, a blower may be required in order to provide adequatepressurization of the recycled anode exhaust flow 42. While FIGS. 1, 2Aand 2B are section views, it will be seen in the later figures that thecomponents and features of the integrated unit 10 are symmetrical aboutthe axis 16, with the exception of the ports 34, 36 and 37.

With reference to FIG. 1 and FIG. 2A, the cathode flows will beexplained in greater detail. As seen in FIG. 1, a cathode feed(typically air), shown schematically by arrows 44, enters the unit 10via the port 34 and passes through an annular passage 46 before enteringa radial passage 48. It should be noted that as used herein, the term“radial passage” is intended to refer to a passage wherein a flow isdirected either radially inward or radially outward in a generallysymmetric 360 degree pattern. The cathode feed 44 flows radially outwardthrough the passage 48 to an annular passage 50 that surrounds the array12 and passes through the cathode recuperator 20. The cathode feed 44flows downward through the annular passage 50 and then flows radiallyinward to an annular feed manifold volume 52 that surrounds the annulararray 12 to distribute the cathode feed 44 into each of the fuel cellstacks 14 where the cathode feed provides oxygen ions for the reactionin the fuel cell stacks 14 and exits the fuel cell stacks 14 as acathode exhaust 56. The cathode exhaust 56 then flows across thereformer 24 into an annular exhaust manifold area 58 where it mixes withthe combustion gas flow 40 which is directed into the manifold 58 via anannular passage 60. In this regard, it should be noted that thecombustion gas flow 40 helps to make up for the loss of mass in thecathode exhaust flow 56 resulting from the transport of oxygen in thefuel cell stacks 14. This additional mass flow provided by thecombustion gas flow 40 helps in minimizing the size of the cathoderecuperator 20. The combined combustion gas flow 40 and cathode exhaust56, shown schematically by arrows 62, exits the manifold 58 via acentral opening 64 to a radial passage 66. The combined exhaust 62 flowsradially outward through the passage 66 to an annular exhaust flowpassage 68 that passes through the cathode recuperator 20 in heatexchange relation with the passage 50 to transfer heat from the combinedexhaust 62 to the cathode feed 44. The combined exhaust 62 flows upwardthrough the annular passage 68 to a radial passage 70 which directs thecombined exhaust 62 radially inward to a final annular passage 72 beforeexiting the unit 10 via the exhaust port 36.

With reference to FIG. 1 and FIG. 2B, an anode feed, shown schematicallyby arrows 80, enters the unit 10 via the anode feed inlet port 30preferably in the form of a mixture of recycled anode exhaust 42 andmethane. The anode feed 80 is directed to an annular passage 82 thatpasses through the anode recuperator 22. The anode feed 80 then flows toa radial flow passage 84 where anode feed 80 flows radially outward toan annular manifold or plenum 86 that directs the anode feed into thereformer 24. After being reformed in the reformer 24, the anode feed 80exits the bottom of reformer 24 as a reformate and is directed into anintegrated pressure plate/anode feed manifold 90. The feed manifold 90directs the anode feed 80 to a plurality of stack feed ports 92, withone of the ports 92 being associated with each of the fuel cell stacks14. Each of the ports 92 directs the anode feed 80 into a correspondinganode feed/return assembly 94 that directs the anode feed 82 into thecorresponding fuel cell stack 14 and collects an anode exhaust, shownschematically by arrows 96, from the corresponding stack 14 after theanode feed reacts in the stack 14. Each of the anode feed/returnassemblies 94 directs the anode exhaust 96 back into a corresponding oneof a plurality of stack ports 98 in the pressure plate/manifold 90(again, one port 98 for each of the fuel cell stacks 14). The manifold90 directs the anode exhaust 96 radially inward to eight anode exhaustports 100 (again, one for each stack 14) that are formed in the pressureplate/manifold 90. The anode exhaust 96 flows through the ports 100 intoa plurality of corresponding anode exhaust tubes 102 which direct theanode exhaust 96 to a radial anode exhaust flow passage 104. The anodeexhaust 96 flows radially inward through the passage 104 to an annularflow passage 106 that passes downward through the anode recuperator 22in heat exchange relation with the flow passage 82. The anode exhaust 96is then directed from the annular passage 106 upward into a tubularpassage 108 by a baffle/cover 110 which is preferably dome-shaped. Theanode exhaust 96 flows upwards through the passage 108 before beingdirected into another annular passage 112 by a baffle/cover 114, whichagain is preferably dome-shaped. The annular passage 112 passes throughthe anode cooler 26 in heat exchange relation with the annular cathodefeed passage 46. After transferring heat to the cathode feed 44, theanode exhaust 96 exits the annular passage 112 and is directed by abaffle 116, which is preferably cone-shaped, into the anode exhaust port32.

With reference to FIGS. 3A, 3B, the reformer 24 is provided in the formof an annular array 280 of eight tube sets 282, with each tube set 282corresponding to one of the fuel cell stacks 14 and including a row offlattened tubes 284. In this regard, it should be noted that the numberof tubes 284 in the tube sets 282 will be highly dependent upon theparticular parameters of each application and can vary from unit 10 tounit 10 depending upon those particular parameters.

FIG. 3C is intended as a generic figure to illustrate certainconstruction details common to the cathode recuperator 20, the anoderecuperator 22, and the anode cooler 26. The construction of each ofthese three heat exchangers basically consists of three concentriccylindrical walls A,B,C that define two separate flow passages D and E,with corrugated or serpentine fin structures G and H provided in theflow passages D and E, respectively, to provide surface areaaugmentation of the respective flow passages. Because the heat transferoccurs through the cylindrical wall B, it is preferred that the fins Gand H be bonded to the wall B in order to provide good thermalconductivity, such as by brazing. On the other hand, for purposes ofassembly and/or allowing differential thermal expansion, it is preferredthat the fins G and H not be bonded to the cylindrical walls A and C.For each of the heat exchangers 20, 22 and 26, it should be understoodthat the longitudinal length and the specific geometry of the fins G andH in each of the flow paths D and E can be adjusted as required for eachparticular application in order to achieve the desired outputtemperatures and allowable pressure drops from the heat exchangers.

Turning now to FIG. 4A-D, the anode cooler 26 includes a corrugated orserpentine fin structure 300 to provide surface area augmentation forthe anode exhaust 96 in the passage 112, a corrugated or serpentine finstructure 302 that provides surface area augmentation for the cathodefeed flow 44 in the passage 46, and a cylindrical wall or tube 304 towhich the fins 300 and 302 are bonded, preferably by brazing, and whichserves to separate the flow passage 46 from the flow passage 112. Asbest seen in FIG. 4B, a cylindrical flow baffle 306 is provided on theinterior side of the corrugated fin 300 and includes the dome-shapedbaffle 114 on its end in order to define the inner part of flow passage112. A donut-shaped flow baffle 308 is also provided to direct thecathode feed 44 radially outward after it exists the flow passage 46.The cone-shaped baffle 116 together with the port 32 are attached to thetop of the tube 304, and include a bolt flange 310 that is structurallyfixed, by a suitable bonding method such as brazing or welding, to theport 32, which also includes a bellows 311 to allow for thermalexpansion between the housing 28 and the components connected throughthe flange 310. As seen in FIG. 4C, the above-described components canbe assembled as yet another subassembly that is bonded together, such asby brazing.

In reference to FIGS. 1 and 4D, it can be seen that the anoderecuperator 22 includes a corrugated or serpentine fin structure 312 inthe annular flow passage 82 for surface area augmentation for anode feed80. As best seen in FIG. 1, the anode recuperator 22 further includesanother corrugated or serpentine fin structure 314 in the annular flowpassage 106 for surface augmentation of the anode exhaust 96.

As best seen in FIG. 4D, corrugated fins 312 and 314 are preferablybonded to a cylindrical wall of tube 316 that serves to separate theflow passages 82 and 106 from each other, with the dome-shaped baffle110 being connected to the bottom end of the wall 316. Anothercylindrical wall or tube 320 is provided radially inboard from thecorrugated fin 314 (not shown in FIG. 4D, but in a location equivalentto fin 300 in cylinder 304 as seen in FIG. 4B) to define the inner sideof the annular passage 106, as best seen in FIG. 4D. As seen in FIG. 2A,an insulation sleeve 322 is provided within the cylindrical wall 320 anda cylindrical exhaust tube 324 is provided within the insulation sleeve322 to define the passage 108 for the anode exhaust 96. Preferably, theexhaust tube 324 is joined to a conical-shaped flange 328 provided at alower end of the cylindrical wall 320. With reference to FIG. 4D,another cylindrical wall or tube 330 surrounds the corrugated fin 312 todefine the radial outer limit of the flow passage 82 and is connected tothe inlet port 30 by a conical-shaped baffle 332. A manifold disk 334 isprovided at the upper end of the wall 316 and includes a central opening336 for receiving the cylindrical wall 320, and eight anode exhaust tubereceiving holes 338 for sealingly receiving the ends of the anodeexhaust tubes 102, with the plate 308 serving to close the upper extentof the manifold plate 334 in the assembled state.

With reference to FIGS. 2B and 4E, a heat shield assembly 350 is shownand includes an inner cylindrical shell 352 (shown in FIG. 2B), an outercylindrical shell 354, an insulation sleeve 356 (shown in FIG. 2B)positioned between the inner and outer shells 352 and 354, and adisk-shaped cover 358 closing an open end of the outer shell 350. Thecover 358 includes eight electrode clearance openings 360 for throughpassage of the electrode sleeves 211. As seen in FIG. 4E, the heatshield assembly 350 is assembled over an insulation disk 361 the outerperimeter of the assembled array 12 of fuel cells 14 and defines theouter extent of the cathode feed manifold 52. The heat shield 350 servesto retain the heat associated with the components that it surrounds.FIG. 5 shows the heat shield assembly 350 mounted over the stacks 14.

With reference to FIG. 1 and FIG. 6, the cathode recuperator 20 includesa corrugated or serpentine fin structure 362 to provide surfaceenhancement in the annular flow passage 68 for the combined exhaust 62,a corrugated or serpentine fin structure 364 to provide surfaceenhancement in the annular flow passage 50 for the cathode feed 44, anda cylindrical tube or wall 366 that separates the flow passages 50 and68 and to which the fins 362 and 364 are bonded. A disk-shaped coverplate 368 is provided to close the upper opening of the cylindrical wall366 and includes a central opening 370, and a plurality of electrodeclearance openings 372 for the passage of the electrode sleeve 211therethrough. A cylindrical tube or sleeve 376 is attached to the cover368 to act as an outer sleeve for the anode cooler 26, and an upperannular bolt flange 378 is attached to the top of the sleeve 376. Alower ring-shaped bolt flange 380 and an insulation sleeve 382 arefitted to the exterior of the sleeve 376, and a cylindrical wall orshield 384 surrounds the insulation sleeve 382 and defines an inner wallfor the passage 72, as best seen in FIGS. 1 and 6.

With reference to FIG. 7, the components of FIG. 6 are then assembledover the components shown in FIG. 5 with the flange 378 being bolted tothe flange 310.

With reference to FIG. 4A, the outer housing 28 is assembled over theremainder of the unit 10 and bolted thereto at flange 380 and a flange400 of the housing 28, and at flange 402 of the assembly 237 and aflange 404 of the housing 28, preferably with a suitable gasket betweenthe flange connections to seal the connections.

FIG. 9 is a schematic representation of the previously describedintegrated unit 10 showing the various flows through the integrated unit10 in relation to each of the major components of the integrated unit10. FIG. 9 also shows an optional air cooled anode condenser 460 that ispreferably used to cool the anode exhaust flow 39 and condense watertherefrom prior to the flow 39 entering the combustor 38. If desired,the condenser may be omitted. FIG. 9 also shows a blower 462 forproviding an air flow to the combustor 38, a blower 464 for providingthe cathode feed 44, and a blower 466 for pressurizing the anode recycleflow 42. If desired, in an alternate embodiment of the unit 10 shown inFIG. 9 also differs from the previously described embodiment shown inFIG. 1 in that an optional steam generator (water/combined exhaust heatexchanger) 440 is added in order to utilize waste heat from the combinedexhaust 62 to produce steam during startup. In this regard, a water flow442 is provided to a water inlet port 444 of the heat exchanger 440, anda steam outlet port directs a steam flow 448 to be mixed with the anodefeed 80 for delivery to the anode feed inlet port 30.

SUMMARY

An embodiment relates to a method of operating a fuel cell systemincluding splitting a cathode exhaust from one or more fuel cell stacksin the system into a majority cathode exhaust stream comprising morethan 50% of the cathode exhaust and a first cathode exhaust bypassstream, providing the majority cathode exhaust stream to an inlet of ananode tail gas oxidizer (ATO) containing a catalyst and providing thefirst cathode bypass stream downstream of the catalyst such that itbypasses the catalyst.

Another embodiment relates to a fuel cell system including at least onefuel cell stack, an anode tail gas oxidizer (ATO) containing a catalyst;an ATO skirt and at least one hole or slit in a lower portion of the ATOdownstream of the catalyst or in the ATO skirt.

Another embodiment relates to a method of operating a fuel cell systemincluding providing an air inlet stream to the SOFC system via a mainair inlet, providing the air inlet stream from the main air inlet to acathode recuperator and providing a cooling medium to a heat exchangerto cool the cathode recuperator.

Another embodiment relates to a fuel cell system comprising a cathoderecuperator, a heat exchanger thermally coupled to the cathoderecuperator and an cooling medium supply configured to provide a coolingmedium to the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a prior art fuel cell unit with anintegrated SOFC and fuel processor.

FIGS. 2A and 2B are sectional views showing one half of the prior artfuel cell unit of FIG. 1, with FIG. 2A illustrating the flows of thecathode feed and exhaust gases and FIG. 2B illustrating the flows of theanode feed and exhaust gases.

FIG. 3A is a sectional view taken from line 3A-3A in FIG. 1, but showingonly selected components of the fuel cell unit.

FIG. 3B is an enlarged, somewhat schematic view taken from line 3B-3B inFIG. 3A.

FIG. 3C is a partial section view illustrating construction detailscommon to several heat exchangers contained within the integrated unitof FIG. 1.

FIGS. 4A and 4B are exploded perspective views of the components of ananode exhaust cooler of the integrated unit of FIG. 1.

FIG. 4C is a perspective view showing the components of FIGS. 4A and Bin their assembled state.

FIG. 4D is an exploded perspective view showing the assembled componentstogether with an anode recuperator of the integrated unit of FIG. 1.

FIG. 4E is an exploded perspective view showing the components of thefuel cell stacks, anode recuperator and anode cooler together with aninsulation disk and heat shield housing of the integrated unit of FIG.1.

FIG. 5 is a perspective view showing the assembled state of thecomponents of FIG. 4E.

FIG. 6 is an exploded perspective view showing a cathode recuperatorassembly together with other components of the integrated unit of FIG.1.

FIG. 7 is an exploded perspective view showing the assembled componentsof FIG. 6 together with the assembled components of FIG. 4.

FIG. 8 is an exploded perspective view showing the assembled componentsof FIG. 7 together with an outer housing of the integrated unit of FIG.1.

FIG. 9 is a schematic representation of the fuel cell unit if FIG. 1.

FIG. 10A is an exploded view of an anode exhaust cooler heat exchangerhaving two finger plates according to an embodiment.

FIG. 10B is a photograph of an exemplary anode exhaust cooler heatexchanger of FIG. 10A.

FIGS. 11A-11H are sectional views of a cathode recuperator according toan embodiment.

FIG. 12 is a sectional view illustrating a uni-shell recuperator locatedon the top of one or more columns of fuel cells according to anembodiment.

FIG. 13 is a three dimensional cut-away view of an anode flow structureaccording to an embodiment.

FIG. 14A is a three dimensional view of an anode hub flow structureaccording to an embodiment.

FIGS. 14B and 14C are side cross sectional views of an anode recuperatoraccording to an embodiment.

FIG. 14D is a top cross sectional view of the anode recuperator of FIGS.17B and 17C.

FIG. 15A is a three dimensional view of an anode tail gas oxidizeraccording to an embodiment.

FIGS. 15B and 15C are three dimensional cut-away views of the anode tailgas oxidizer of FIG. 15A, and FIG. 15D is a top view of the fuel cellsystem.

FIG. 16 is a schematic process flow diagram illustrating a hot boxaccording to an embodiment.

FIG. 17A is a side cross sectional view of an anode recuperatoraccording to an embodiment.

FIG. 17B is a side cross sectional view of an anode recuperatoraccording to another embodiment.

FIG. 18A is a perspective cross sectional view of the anode recuperatorillustrated in FIG. 17A.

FIG. 18B is a top cross sectional view of the anode recuperatorillustrated in FIG. 17A.

FIG. 18C another top cross sectional view of the anode recuperatorillustrated in FIG. 17A.

FIG. 19A is a schematic diagram illustrating a hot box according to anembodiment.

FIG. 19B is a three dimensional cut-away view of an anode flow structureof the embodiment illustrated in FIG. 19A.

FIG. 20 is a schematic process flow diagram illustrating a hot boxutilizing the embodiment illustrated in FIGS. 19A and 19B.

FIGS. 21A and 21B are schematic illustrations of a bimetal louveraccording to an embodiment.

FIG. 22A is a schematic cross-section illustrating cathode flows in aSOFC system without additional cooling to the cathode recuperator.

FIG. 22B is a schematic cross-section illustrating cathode flows in aSOFC system with additional cooling to the cathode recuperator.

FIG. 22C is a plan view illustrating the SOFC system of FIG. 22Baccording to an embodiment.

FIG. 22D is a plan view illustrating the SOFC system of FIG. 22Baccording to another embodiment.

FIG. 22E is a plan view illustrating the SOFC system of FIG. 22Baccording to another embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The inventors have discovered that raising the ATO temperature withoutburning any additional fuel improves the performance of SOFC systems. Inan embodiment, this is achieved by removing some of the cathode exhaustfrom the combustion section of the ATO. In an embodiment, this bypass isaccomplished by forming holes or slits in the bottom of the outer ATOcylinder. In this embodiment, combustion may be performed (includingnearly complete conversion of CO) at or near 900 C, beforecooling/diluting the ATO exhaust with cooler air. In an alternativeembodiment, the holes or slits are formed in the ATO skirt, allowing aportion of cathode exhaust to enter the manifold outside the spider hub.

Additionally, as the power level of a hot box increases, so does theamount of heat that needs to be removed. Conventionally, there are twomethods to remove heat from the hot box: (1) conductive losses throughthe hot box insulation to the cabinet air and (2) increasing thesensible heat of the exhaust stream.

Regarding the first method, when the stacks are at temperature,conductive losses are typically ˜4 kW and are independent of the powerlevel. However, conductive losses cannot be scaled up as the power levelincreases.

Regarding the second method, to maintain power as the stacks degrade,current is increased. This increase in current leads to an increase infuel flow and main air flow, thereby increasing the exhaust flow. Sincethe cathode recuperator is a fixed size, the increased flow also leadsto an increased cathode exhaust outlet temperature. This method ofrejecting heat is scalable, but has multiple disadvantages. For example,high exhaust temperatures are a problem for the enclosure ventilationsystem. The ventilation fan must be capable of withstanding anynonuniform thermal transients. Thus, the amount of ventilation air hasto be increased to “dilute” the heat down to a low enough temperaturefor discharge. Further, increasing the air flow increases the parasiticpower demand of the main air blower. The pressure drop of the balance ofplant (BOP) and hot box components that pass main air or exhaust alsoincreases, further increasing the power demand. In an embodiment, theSOFC system includes a chromium oxide (co) scrubber. This componentworks best at cooler exhaust temperatures. However, cooling the exhaustby diluting it with cabinet air is not practical, as the co scrubber hasa residence time requirement and imposes a significant pressure drop.

Anode Exhaust Cooler Heat Exchanger

It is desirable to increase overall flow conditions and rates of thefluids (e.g., fuel and air inlet and exhaust streams) in the hot box.According to the first embodiment, an anode exhaust cooler heatexchanger with “finger plates” facilitates these higher overall flowconditions. An anode cooler heat exchanger is a heat exchanger in whichthe hot fuel exhaust stream from a fuel cell stack exchanges heat with acool air inlet stream being provided to the fuel cell stack (such as aSOFC stack). This heat exchanger is also referred to as an airpre-heater heat exchanger in U.S. application Ser. No. 12/219,684 filedon Jul. 25, 2008 and Ser. No. 11/905,477 filed on Oct. 1, 2007, both ofwhich are incorporated herein by reference in their entirety.

An exemplary anode exhaust cooler heat exchanger 100 is illustrated inFIGS. 10A-10B and 11. Embodiments of the anode exhaust cooler heatexchanger 100 include two “finger” plates 102 a, 102 b sealed onopposite ends of a corrugated sheet 104, as shown in FIG. 10A. Thecorrugated sheet 104 may have a cylindrical shape (i.e., a cylinder witha corrugated outer wall) and the finger plates 102 a, 102 b are locatedon the opposite ends of the cylinder. That is, the peaks and valleys ofthe corrugations may be aligned parallel to the axial direction of thecylinder with the finger plates 102 a, 102 b designed to coveralternating peaks/valleys. Other shapes (e.g., hollow rectangle,triangle or any polygon) are also possible for the sheet 104. The fingerplates comprise hollow ring shaped metal plates which have finger shapedextensions which extend into the inner portion of the ring. The plates102 a, 102 b are offset from each other by one corrugation, such that ifthe fingers of top plate 102 a cover every inward facing recess in sheet104, then bottom plate 102 b fingers cover every outward facing recessin sheet 104 (as shown in FIG. 10B which illustrates an assembled heatexchanger 100), and vise-versa. The shape of each finger is configuredto cover one respective recess/fin/corrugation in sheet 104. The fingersmay be brazed to the sheet 104.

The corrugations or fins of the sheet 104 may be straight as shown inFIG. 10A or wavy as shown in FIG. 10B. The wavy fins are fins which arenot straight in the vertical direction. Such wavy fins are easier tomanufacture.

Embodiments of the anode exhaust cooler heat exchanger may have one ormore of the following advantages: excellent heat exchange due to minimalmaterial conduction losses between separated flow streams, very compact,light weight, reduced material requirements, reduced manufacturingcosts, elimination of fixture requirements, reduced pressure drop,ability to control flow ratios between two or more flow streams bysimply changing finger plate design. The duty of the anode exhaustcooler heat exchanger may be increased by 20-40% over the prior art heatexchanger. Further, in some embodiments, the anode exhaust cooler heatexchanger may also be shorter than the prior art heat exchanger inaddition to having a higher duty.

Cathode Recuperator Uni-shell

The cathode recuperator is a heat exchanger in which the air inletstream exchanges heat with the air (e.g., cathode) exhaust stream fromthe fuel cell stack. Preferably, the air inlet stream is preheated inthe anode cooler described above before entering the cathoderecuperator.

The mode of heat transfer through the prior art brazed two finnedcylindrical heat exchanger is defined by that amount of conductive heattransfer that is possible through the brazed assembly of the heatexchange structure. The potential lack of heat transfer can causethermal instability of the fuel cell system and also may not allow thesystem to operate at its rated conditions. The inventors realized thatthe use of a single fin flow separator improves the heat transferbetween fluid streams and provides for a compact heat exchanger package.

An example cathode recuperator 200 uni-shell is illustrated in FIGS. 11Ato 11G. In an embodiment, the three concentric and independent shells A,B and C of FIG. 3C of the prior art structure replaced with a singlemonolithic assembly shown in FIGS. 11A-11B. FIG. 11A shows an explodedthree dimensional view of the assembly components without the heatshield insulation and FIGS. 11B and 11C show three dimensional views ofthe assembly with the components put together and the heat shieldinsulation 202A, 202B installed.

Embodiments of the uni-shell cathode recuperator 200 include a singlecylindrical corrugated fin plate or sheet 304 (shown in FIGS. 11A and11D). The corrugated plate or sheet 304 is preferably ring shaped, suchas hollow cylinder. However, plate or sheet 304 may have a polygonalcross section when viewed from the top if desired. The corrugated plateor sheet 304 is located between inner 202A and outer 202B heat shieldinsulation as shown in FIG. 11C, which is a three dimensional view ofthe middle portion of the recuperator 200, FIG. 11D which is a top viewof the plate or sheet 304, and the FIG. 11E which is a side crosssectional view of the recuperator 200. The heat shield insulation maycomprise hollow cylinders. The heat shield insulation may be supportedby a heat shield shell 204 located below the corrugated plate or sheet304.

In addition to the insulation and the corrugated plate or sheet 304, theuni-shell cathode recuperator 200 also includes a top cap, plate or lid302 a (shown in FIG. 11A) and a similar bottom cap plate or lid (notshown in FIG. 12A for clarity). As shown in FIGS. 11A, 11B, 11F and 11G,in addition to the top cap, plate or lid 302 a, the hot box may alsoinclude a heat shield 306 with support ribs below lid 302 a, a steamgenerator 103 comprising a baffle plate 308 with support ribs supportinga steam coil assembly 310 (i.e., the coiled pipe through which flowingwater is heated to steam by the heat of the air exhaust stream flowingaround the pipe), and an outer lid 312 with a weld ring 313 enclosingthe steam generator 103. A cathode exhaust conduit 35 in outer lid 312exhausts the air exhaust stream from the hot box.

The single cylindrical corrugated fin plate 304 and top and bottom capplates force the air (i.e., cathode) inlet 12314 and air (i.e., cathode)exhaust streams 1227 to make a non-zero degree turn (e.g., 20-160 degreeturn, such as a 90 degree) turn into adjoining hollow fins of the finplate 304 as shown in FIGS. 11F (side cross sectional view of theassembly) and 11G (three dimensional view of the assembly). For example,the cathode or air inlet stream flows from the anode cooler 100 to thecathode recuperator 200 through conduit 314 which is located between theheat shield 306 and the top cap 302 a. The air inlet stream flowssubstantially horizontally in an outward radial direction (i.e., in toout radially) as shown by the arrows in FIGS. 11F and 11G until thestream impacts the inner surface of the upper portion of the corrugatedfin plate 304. The impact forces the stream to make a 90 degree turn andflow down (i.e., in an axial direction) in the inner corrugations.Likewise, the hot cathode exhaust stream shown by arrows in FIGS. 11Fand 11G first flows vertically from below through conduit 27 from theATO and is then substantially horizontally in the end portion of conduit27 in a substantially inward radial direction to impact the outersurface of the lower portions of the corrugated fin plate 304. Thiscauses the air exhaust stream to make a non-zero degree turn and flow up(i.e., in an axial direction) in the outer corrugations of plate 304.This single layer fin plate 304 design allows for effective heattransfer and minimizes the thermal variation within the system (from themisdistribution of air).

The use of the cap plates in the cathode recuperator is not required.The same function could be achieved with the use of finger platessimilar to finger plates 102 a, 102 b illustrated for the anode cooler100. The cathode recuperator heat exchanger 200 may be fabricated witheither the finger plates or the end caps located on either end or acombination of both. In other words, for the combination of finger plateand end cap, the top of the fin plate 304 may contain one of fingerplate or end cap, and the bottom of the fins may contain the other oneof the finger plate or end cap

Hot and cold flow streams flow in adjacent corrugations, where the metalof the corrugated plate or sheet 304 separating the flow streams acts asa primary heat exchanger surface, as shown in FIG. 11D, which is a topcross sectional view of a portion of plate or sheet 304. For example,the relatively cool or cold air inlet stream 12314 flows inside of thecorrugated plate or sheet 304 (including in the inner recesses of thecorrugations) and the relatively warm or hot air exhaust stream 1227flows on the outside of the plate or sheet 304 (including the outerrecesses of the corrugations). Alternatively, the air inlet stream 12314may flow on the inside and the air exhaust stream 1227 may flow on theoutside of the corrugated plate or sheet 304.

One side (e.g., outer side) of the corrugated plate or sheet 304 is influid communication with an air exhaust conduit 27 which is connected tothe air exhaust of the solid oxide fuel cell stack and/or the ATOexhaust. The second side of the corrugated plate or sheet 304 is influid communication with a warm air output conduit 314 of the anodecooler 100 described above.

As shown in FIG. 11H, the air inlet stream 1225 exiting the cathoderecuperator 200 may be directed towards the middle lengthwise portion ofa fuel cell stack or column 9 to provide additional cooling in theotherwise hottest zone of the stack or column 9. In other words, middleportion of the fuel cell stack or column 9 is relatively hotter than thetop and bottom end portions. The middle portion may be located betweenend portions of the stack or column 9 such that each end portion extends10-25% of the length of the stack or column 9 and the middle portion is50-80% of the length of the stack or column 9.

The location of the air inlet stream outlet 210 of the recuperator 200can be tailored to optimize the fuel cell stack or column 9 temperaturedistributions. Thus, the vertical location of outlet 210 may be adjustedas desired with respect to vertically oriented stack or column 9. Theoutlet 210 may comprise a circular opening in a cylindrical recuperator200, or the outlet 210 may comprise one or more discreet openingsadjacent to each stack or column 9 in the system.

Since the air inlet stream (shown by dashed arrow in FIG. 11H) exitingoutlet 210 is relatively cool compared to the temperature of the stackor column 9, the air inlet stream may provide a higher degree of coolingto the middle portion of the stack or column compared to the endportions of the stack or column to achieve a higher temperatureuniformity along the length of the stack of column. For example, theoutlet 210 may be located adjacent to any one or more points in themiddle 80%, such as the middle 50%, such as the middle 33% of the stackor column. In other words, the outlet 210 is not located adjacent toeither the top or bottom end portions each comprising 10%, such as 25%such as 16.5% of the stack or column.

Embodiments of the uni-shell cathode recuperator 200 may have one ormore of the following advantages: excellent heat exchange due to minimalmaterial conduction losses between separated flow streams, very compact,light weight, reduced material requirements, reduced manufacturingcosts, reduced pressure drop, provides dead weight as insurance formechanical compression failure. This allows for easier assembly of thefuel cell system, reduced tolerance requirements and easiermanufacturing of the assembly.

Thus, as described above, the anode cooler 100 and the cathoderecuperator 200 comprise “uni-shell” heat exchangers where the processgases flow on the two opposing surfaces of a roughly cylindricalcorrugated sheet. This provides a very short conductive heat transferpath between the streams. The hotter stream (e.g., anode exhaust and ATOexhaust streams in heat exchangers 100, 200, respectively) providesconvective heat transfer to a respective large surface area corrugatedmetal separator sheet 104, 304. Conductive heat transfer then proceedsonly through the small thickness of the separator (e.g., the thicknessof the corrugated sheet 104, 304), and then convective heat transfer isprovided from the sheet 104, 304 to the cooler respective stream (e.g.,the air inlet stream in both heat exchangers 100, 200).

The heat exchangers 100, 200 differ in their approach to manifoldingtheir respective process streams. The roughly cylindrical anode cooler100 uses finger shaped apertures and finger plates 102 a, 102 b to allowa substantially axial entry of the process streams (i.e., the anodeexhaust and air inlet streams) into the corrugated cylindrical sectionof the heat exchanger. In other words, the process streams enter theheat exchanger 100 roughly parallel (e.g., within 20 degrees) to theaxis of the roughly cylindrical heat exchanger.

In contrast, the cathode recuperator 200 includes top and bottom caps302 a, which require the process streams (e.g., the air inlet stream andATO exhaust stream) to enter the heat exchanger 200 roughlyperpendicular (e.g., within 20 degrees) to the axial direction of theheat exchanger 200. Thus, heat exchanger 200 has a substantiallynon-axial process gas entry into the heat exchanger.

If desired, these manifolding schemes may be switched. Thus, both heatexchangers 100, 200 may be configured with the axial process gas entryor non-axial process gas entry. Alternatively, heat exchanger 200 may beconfigured with the axial process gas entry and/or heat exchanger 100may be configured with non-axial process gas entry.

Cathode Recuperator Uni-shell with Ceramic Column Support

In the prior fuel cell systems, it is difficult to maintain a continuousmechanical load on the fuel cell stacks or columns of stacks through thefull range of thermal operating conditions. To maintain a mechanicalload, the prior art systems rely on an external compression system.Embodiments of the present fuel cell system do not include an externalcompression system. The removal of the external compression system,however, can lead to a loss of mechanical integrity of the fuel cellcolumns. The inventors have realized, however, that the externalcompression system can be replaced by an internal compression systemcomprising either a spring loaded or gravity loaded system or acombination of both. The spring loaded system may comprise any suitablesystem, such as a system described U.S. patent application Ser. No.12/892,582 filed on Sep. 28, 2010 and which is incorporated herein byreference in its entirety, which describes an internal compressionceramic spring, and/or or use the uni-shell bellow in conjunction withappropriately tailored thermal expansion of the column and uni-shellmaterial.

In an embodiment shown in FIG. 12, the uni-shell cathode recuperator 200is located on top of one or more columns 402 to provide additionalinternal compression for the stack or column of stacks 9. The weight ofthe recuperator 200 uni-shell cylinder(s) can act directly on the fuelcell columns 9. With the added weight of the cylinders, the fuel cellcolumns can be prevented from lifting off the hot box base 500 andprovide any required sealing forces. Any suitable columns 402 may beused. For example, the ceramic columns 402 described in U.S. applicationSer. No. 12/892,582 filed on Sep. 28, 2010 and which is incorporatedherein by reference in its entirety may be used.

As discussed in the above described application, the ceramic columns 402comprise interlocked ceramic side baffle plates 402A, 402B, 402C. Thebaffle plates may be made from a high temperature material, such asalumina, other suitable ceramic, or a ceramic matrix composite (CMC).The CMC may include, for example, a matrix of aluminum oxide (e.g.,alumina), zirconium oxide or silicon carbide. Other matrix materials maybe selected as well. The fibers may be made from alumina, carbon,silicon carbide, or any other suitable material. Any combination of thematrix and fibers may be used. The ceramic plate shaped baffle platesmay be attached to each other using dovetails or bow tie shaped ceramicinserts as described in the Ser. No. 12/892,582 application.Furthermore, as shown in FIG. 12, one or more fuel manifolds 404 may beprovided in the column of fuel cell stacks 9, as described in the Ser.No. 12/892,582 application.

Furthermore, an optional spring compression assembly 406 may be locatedover the fuel cell column 9 and link adjacent ceramic columns 402 whichare located on the opposing sides of the column of fuel cell stacks 9.The assembly 406 may include a ceramic leaf spring or another type ofspring between two ceramic plates and a tensioner, as described in theSer. No. 12/892,582 application. The uni-shell cathode recuperator 200may be located on a cap 408 on top of the assembly 406, which providesinternal compression to the ceramic columns 402 and to the column offuel cell stacks 9.

Embodiments of the recuperator uni-shell may have one or more of thefollowing advantages: improved sealing of air bypass at the top of thecolumns and continuous load on the columns. The continuous load on thecolumns gives some insurance that even with failure of the internalcompression mechanism there would still be some (vertical) mechanicalload on the columns. The use of the expansion bellows 206 within theuni-shell assembly allows for the shell assembly to expand and contractindependently from the main anode flow structure of the system, therebyminimizing the thermo-mechanical effects of the two subassemblies.

Anode Flow Structure and Flow Hub

FIG. 13 illustrates the anode flow structure according to one embodimentof the invention. The anode flow structure includes a cylindrical anoderecuperator (also referred to as a fuel heat exchanger)/pre-reformer137, the above described anode cooler (also referred to as an airpre-heater) heat exchanger 100 mounted over the anode recuperator, andan anode tail gas oxidizer (ATO) 10.

The ATO 10 comprises an outer cylinder 10A which is positioned aroundthe inner ATO insulation 10B/outer wall of the anode recuperator 137.Optionally, the insulation 10B may be enclosed by an inner ATO cylinder10D, as shown in FIG. 18B. Thus, the insulation 10B is located betweenthe outer anode recuperator cylinder and the inner ATO cylinder 10D. Anoxidation catalyst 10C is located in the space between the outercylinder 10A and the ATO insulation 10B (or inner ATO cylinder 10D ifpresent). An ATO thermocouple feed through 1601 extends through theanode exhaust cooler heat exchanger 100 and the cathode recuperator 200to the top of the ATO 10. The temperature of the ATO may thereby bemonitored by inserting a thermocouple (not shown) through this feedthrough 1601.

An anode hub structure 600 is positioned under the anode recuperator 137and ATO 10 and over the hot box base 500. The anode hub structure iscovered by an ATO skirt 1603. A combined ATO mixer 801/fuel exhaustsplitter 107 is located over the anode recuperator 137 and ATO 10 andbelow the anode cooler 100. An ATO glow plug 1602, which aids theoxidation of the stack fuel exhaust in the ATO, may be located near thebottom of the ATO. Also illustrated in FIG. 13 is a lift base 1604 whichis located under the fuel cell unit. In an embodiment, the lift base1604 includes two hollow arms with which the forks of a fork truck canbe inserted to lift and move the fuel cell unit, such as to remove thefuel cell unit from a cabinet (not shown) for repair or servicing.

FIG. 14A illustrates an anode flow hub structure 600 according to anembodiment. The hub structure 600 is used to distribute fuel evenly froma central plenum to plural fuel cell stacks or columns. The anode flowhub structure 600 includes a grooved cast base 602 and a “spider” hub offuel inlet pipes 21 and outlet pipes 23A. Each pair of pipes 21, 23Aconnects to one of the plurality of stacks or columns. Anode sidecylinders (e.g., anode recuperator 137 inner and outer cylinders and ATOouter cylinder 10A) are then welded or brazed into the grooves in thebase 602 creating a uniform volume cross section for flow distribution,as shown in FIGS. 17B, 17C and 18, respectively. The “spider” fuel tubes21, 23A run from the anode flow hub 600 out to the stacks where they arewelded to vertical fuel rails (see e.g., element 94 in FIG. 1). Theanode flow hub 600 may be created by investment casting and machiningand is greatly simplified over the prior art process of brazing largediameter plates.

As shown in FIGS. 14B and 14C (side cross sectional views) and 14D (topcross sectional view) the anode recuperator 137 includes an innercylinder 139, a corrugated finger plate or cylinder 137B and an outercylinder 137C coated with the ATO insulation 10B. FIG. 14B shows thefuel inlet flow 1729 from fuel inlet conduit 29 which bypasses the anodecooler 100 through its hollow core, then between the cylinders 139 and137B in the anode recuperator 137 and then to the stacks or columns 9(flow 1721) (shown also in FIG. 16) through the hub base 602 andconduits 21. FIG. 14C shows the fuel exhaust flow 1723A from the stacksor columns 9 through conduits 23A into the hub base 602, and from thehub base 602 through the anode recuperator 137 between cylinders 137Band 137C into the splitter 107. One part of the fuel exhaust flow streamfrom the splitter 107 flows through the above described anode cooler 100while another part flows from the splitter 107 into the ATO 10. Anodecooler inner core insulation 100A may be located between the fuel inletconduit 29 and the bellows 852/supporting cylinder 852A located betweenthe anode cooler 100 and the ATO mixer 801, as shown in FIGS. 13, 14Band 14C. This insulation minimizes heat transfer and loss from the anodeexhaust stream in conduit 31 on the way to the anode cooler 100.Insulation 100A may also be located between conduit 29 and the anodecooler 100 to avoid heat transfer between the fuel inlet stream inconduit 29 and the streams in the anode cooler 100. Furthermore,additional insulation may be located around the bellows 852/cylinder852A (i.e., around the outside surface of bellows/cylinder) if desired.

FIG. 14C also shows the air inlet flow from conduit 33 through the anodecooler 100 (where it exchanges heat with the fuel exhaust stream) andinto the cathode recuperator 200 described above.

Embodiments of the anode flow hub 600 may have one or more of thefollowing advantages: lower cost manufacturing method, ability to usefuel tube in reformation process if required and reduced fixturing.

ATO Air Swirl Element

In another embodiment of the invention, the present inventors realizedthat in the prior art system shown in FIGS. 1-9, the azimuthal flowmixing could be improved to avoid flow streams concentrating hot zonesor cold zones on one side of the hot box 1. Azimuthal flow as usedherein includes flow in angular direction that curves away in aclockwise or counterclockwise direction from a straight linerepresenting a radial direction from a center of a cylinder to an outerwall of the cylinder, and includes but is not limited to rotating,swirling or spiraling flow. The present embodiment of the inventionprovides a vane containing swirl element for introducing swirl to theair stream provided into the ATO 10 to promote more uniform operatingconditions, such as temperature and composition of the fluid flows.

As shown in FIGS. 15A, 15B and 15C, one embodiment of an ATO mixer 801comprises a turning vane assembly which moves the stack air exhauststream heat azimuthally and/or radially across the ATO to reduce radialtemperature gradients. The cylindrical mixer 801 is located above theATO 10 and may extend outwardly past the outer ATO cylinder 10A.Preferably, the mixer 801 is integrated with the fuel exhaust splitter107 as will be described in more detail below.

FIG. 15B is a close up, three dimensional, cut-away cross sectional viewof the boxed portion of the ATO 10 and mixer 801 shown in FIG. 15A. FIG.15C is a three dimensional, cut-away cross sectional view of theintegrated ATO mixer 801/fuel exhaust splitter 107.

As shown in FIG. 15A, the turning vane assembly ATO mixer 801 maycomprise two or more vanes 803 (which may also be referred to asdeflectors or baffles) located inside an enclosure 805. The enclosure805 is cylindrical and contains inner and outer surfaces 805A, 805B,respectively (as shown in FIG. 15C), but is generally open on top toreceive the cathode exhaust flow from the stacks 9 via air exhaustconduit or manifold 24. The vanes 803 may be curved or they may bestraight. A shape of turning vane 803 may curve in a golden ratio arc orin catenary curve shape in order to minimize pressure drop per rotationeffect.

The vanes 803 are slanted (i.e., positioned diagonally) with respect tothe vertical (i.e., axial) direction of the ATO cylinders 10A, 10D, atan angle of 10 to 80 degrees, such as 30 to 60 degrees, to direct thecathode exhaust 1824 in the azimuthal direction. At the base of eachvane 803, an opening 807 into the ATO 10 (e.g., into the catalyst 10Ccontaining space between ATO cylinders 10A and 10D) is provided. Theopenings 807 provide the cathode exhaust 1824 azimuthally from theassembly 801 into the ATO as shown in FIG. 15C. While the assembly 801is referred to as turning vane assembly, it should be noted that theassembly 801 does not rotate or turn about its axis. The term “turning”refers to the turning of the cathode exhaust stream 1824 in theazimuthal direction.

The assembly 801 may comprise a cast metal assembly. Thus, the air exitsthe fuel cell stacks it is forced to flow downwards into the ATO mixer801. The guide vanes 803 induce a swirl into the air exhaust stream 1824and direct the air exhaust stream 1824 down into the ATO. The swirlcauses an averaging of local hot and cold spots and limits the impact ofthese temperature maldistributions. Embodiments of the ATO air swirlelement may improve temperature distribution which allows all stacks tooperate at closer points, reduced thermal stress, reduced componentdistortion, and longer operating life.

ATO Fuel Mixer/Injector

Prior art systems include a separate external fuel inlet stream into theATO. One embodiment of the present provides a fuel exhaust stream as thesole fuel input into the ATO. Thus, the separate external ATO fuel inletstream can be eliminated.

As will be described in more detail below and as shown in FIGS. 14C and15C, the fuel exhaust stream 1823B exiting the anode recuperator 137through conduit 23B is provided into splitter 107. The splitter 107 islocated between the fuel exhaust outlet conduit 23B of the anoderecuperator 137 and the fuel exhaust inlet of the anode cooler 100(e.g., the air pre-heater heat exchanger). The splitter 107 splits thefuel exhaust stream into two streams. The first stream 18133 is providedto the ATO 10. The second stream is provided via conduit 31 into theanode exhaust cooler 100.

The splitter 107 contains one or more slits or slots 133 shown in FIGS.15B and 15C, to allow the splitter 107 functions as an ATO fuelinjector. The splitter 107 injects the first fuel exhaust stream 18133in the ATO 10 through the slits or slots 133. A lip 133A below the slits133 and/or the direction of the slit(s) force the fuel into the middleof the air exhaust stream 1824 rather than allowing the fuel exhauststream to flow along the ATO wall 10A or 10D. Mixing the fuel with theair stream in the middle of the flow channel between ATO walls 10A and10D allows for the highest temperature zone to be located in the flowstream rather than on the adjacent walls. The second fuel exhaust streamwhich does not pass through the slits 133 continues to travel upwardinto conduit 31, as shown in FIG. 14C. The amount of fuel exhaustprovided as the first fuel exhaust stream into the ATO through slits 133versus as the second fuel exhaust stream into conduit 31 is controlledby the anode recycle blower 123 speed (see FIGS. 14C and 16). The higherthe blower 123 speed, the larger portion of the fuel exhaust stream isprovided into conduit 31 and a smaller portion of the fuel exhauststream is provided into the ATO 10, and vice-versa.

Alternate embodiments of the ATO fuel injector include porous media,shower head type features, and slits ranging in size and geometry.

Preferably, as shown in FIG. 15C, the splitter 107 comprises an integralstructure with the ATO mixer 801. The slits 133 of the splitter arelocated below the vanes 803 such that the air exhaust stream which isazimuthally rotated by the vanes while flowing downward into the ATO 10provides a similar rotation to the first fuel exhaust stream passingthrough the slits 133 into air exhaust steam in the ATO. Alternatively,the splitter 107 may comprise a brazed on ring which forms the ATOinjector slit 133 by being spaced apart from its supporting structure.

Cathode Exhaust Swirl Element

Stacks could also be rotated slightly on their axis such that the facesof the stacks which face the middle of the ring of stacks do not alignradially, but are positioned with respect to each other at a slight,non-zero angle, such as 1 to 20 degrees for example. This may create aslight swirl to the cathode exhaust stream (i.e., air) leaving thestacks moving in towards the central axis of the hot box. The advantageof this swirl effect is the blending of cathode exhaust temperaturesfrom column to column resulting in more uniform temperaturedistribution. FIG. 15D illustrates the top view of the fuel cell systemof FIG. 3A where the stacks 14 are rotated such that the faces 14 a ofthe stacks which face the middle of the ring of stacks of the stacks donot align radially. In other words, the faces 14 a shown by dashed linesare not tangential to the circle which forms the interior of the ring ofstacks 14, but deviate from the tangent by 1-20 degrees.

Process Flow Diagram

FIG. 16 is a schematic process flow diagram representation of the hotbox 1 components showing the various flows through the componentsaccording to another embodiment of the invention. The components in thisembodiment may have the configuration described in the prior embodimentsor a different suitable configuration. In this embodiment, there are nofresh fuel and fresh air inputs to the ATO 10.

Thus, in contrast to the prior art system, external natural gas oranother external fuel is not fed to the ATO 10. Instead, the hot fuel(anode) exhaust stream from the fuel cell stack(s) 9 is partiallyrecycled into the ATO as the ATO fuel inlet stream. Likewise, there isno outside air input into the ATO. Instead, the hot air (cathode)exhaust stream from the fuel cell stack(s) 9 is provided into the ATO asthe ATO air inlet stream.

Furthermore, the fuel exhaust stream is split in a splitter 107 locatedin the hot box 1. The splitter 107 is located between the fuel exhaustoutlet of the anode recuperator (e.g., fuel heat exchanger) 137 and thefuel exhaust inlet of the anode cooler 100 (e.g., the air pre-heaterheat exchanger). Thus, the fuel exhaust stream is split between themixer 105 and the ATO 10 prior to entering the anode cooler 100. Thisallows higher temperature fuel exhaust stream to be provided into theATO than in the prior art because the fuel exhaust stream has not yetexchanged heat with the air inlet stream in the anode cooler 100. Forexample, the fuel exhaust stream provided into the ATO 10 from thesplitter 107 may have a temperature of above 350 C, such as 350-500 C,for example 375 to 425 C, such as 390-410 C. Furthermore, since asmaller amount of fuel exhaust is provided into the anode cooler 100(e.g., not 100% of the anode exhaust is provided into the anode coolerdue to the splitting of the anode exhaust in splitter 107), the heatexchange area of the anode cooler 100 described above may be reduced.

The splitting of the anode exhaust in the hot box prior to the anodecooler has the following benefits: reduced cost due to the smaller heatexchange area for the anode exhaust cooler, increased efficiency due toreduced anode recycle blower 123 power, and reduced mechanicalcomplexity in the hot box due to fewer fluid passes.

The benefits of eliminating the external ATO air include reduced costsince a separate ATO fuel blower is not required, increased efficiencybecause no extra fuel consumption during steady state or ramp to steadystate is required, simplified fuel entry on top of the hot box next toanode gas recycle components, and reduced harmful emissions from thesystem because methane is relatively difficult to oxidize in the ATO. Ifexternal methane/natural gas is not added to the ATO, then it cannotslip.

The benefits of eliminating the external ATO fuel include reduced costbecause a separate ATO air blower is not required and less ATOcatalyst/catalyst support is required due to higher average temperatureof the anode and cathode exhaust streams compared to fresh external fueland air streams, a reduced cathode side pressure drop due to lowercathode exhaust flows, increased efficiency due to elimination of thepower required to drive the ATO air blower and reduced main air blower125 power due to lower cathode side pressure drop, reduced harmfulemissions since the ATO operates with much more excess air, andpotentially more stable ATO operation because the ATO is always hotenough for fuel oxidation after start-up.

The hot box 1 contains the plurality of the fuel cell stacks 9, such asa solid oxide fuel cell stacks (where one solid oxide fuel cell of thestack contains a ceramic electrolyte, such as yttria stabilized zirconia(YSZ) or scandia stabilized zirconia (SSZ), an anode electrode, such asa nickel-YSZ or Ni-SSZ cermet, and a cathode electrode, such aslanthanum strontium manganite (LSM)). The stacks 9 may be arranged overeach other in a plurality of columns as shown in FIG. 13A.

The hot box 1 also contains a steam generator 103. The steam generator103 is provided with water through conduit 30A from a water source 104,such as a water tank or a water pipe (i.e., a continuous water supply),and converts the water to steam. The steam is provided from generator103 to mixer 105 through conduit 30B and is mixed with the stack anode(fuel) recycle stream in the mixer 105. The mixer 105 may be locatedinside or outside the hot box of the hot box 1. Preferably, thehumidified anode exhaust stream is combined with the fuel inlet streamin the fuel inlet line or conduit 29 downstream of the mixer 105, asschematically shown in FIG. 16. Alternatively, if desired, the fuelinlet stream may also be provided directly into the mixer 105, or thesteam may be provided directly into the fuel inlet stream and/or theanode exhaust stream may be provided directly into the fuel inlet streamfollowed by humidification of the combined fuel streams.

The steam generator 103 is heated by the hot ATO 10 exhaust stream whichis passed in heat exchange relationship in conduit 119 with the steamgenerator 103, as shown in FIG. 11F.

The system operates as follows. The fuel inlet stream, such as ahydrocarbon stream, for example natural gas, is provided into the fuelinlet conduit 29 and through a catalytic partial pressure oxidation(CPOx) 111 located outside the hot box. During system start up, air isalso provided into the CPOx reactor 111 through CPOx air inlet conduit113 to catalytically partially oxidize the fuel inlet stream. Duringsteady state system operation, the air flow is turned off and the CPOxreactor acts as a fuel passage way in which the fuel is not partiallyoxidized. Thus, the hot box 1 may comprise only one fuel inlet conduitwhich provides fuel in both start-up and steady state modes through theCPOx reactor 111. Therefore a separate fuel inlet conduit which bypassesthe CPOx reactor during steady state operation is not required.

The fuel inlet stream is provided into the fuel heat exchanger (anoderecuperator)/pre-reformer 137 where its temperature is raised by heatexchange with the stack 9 anode (fuel) exhaust streams. The fuel inletstream is pre-reformed in the pre-reformer section of the heat exchanger137 via the SMR reaction and the reformed fuel inlet stream (whichincludes hydrogen, carbon monoxide, water vapor and unreformed methane)is provided into the stacks 9 through the fuel inlet conduit(s) 21.Additional reformation catalyst may be located in conduit(s) 21. Thefuel inlet stream travels upwards through the stacks through fuel inletrisers in the stacks 9 and is oxidized in the stacks 9 duringelectricity generation. The oxidized fuel (i.e., the anode or fuelexhaust stream) travels down the stacks 9 through the fuel exhaustrisers and is then exhausted from the stacks through the fuel exhaustconduits 23A into the fuel heat exchanger 137.

In the fuel heat exchanger 137, the anode exhaust stream heats the fuelinlet stream via heat exchange. The anode exhaust stream is thenprovided via the fuel exhaust conduit 23B into a splitter 107. A firstportion of the anode exhaust stream is provided from the splitter 107the ATO 10 via conduit (e.g., slits) 133.

A second portion of the anode exhaust stream is recycled from thesplitter 107 into the anode cooler 100 and then into the fuel inletstream. For example, the second portion of the anode exhaust stream isrecycled through conduit 31 into the anode cooler (i.e., air pre-heaterheat exchanger) where the anode exhaust stream pre-heats the air inletstream from conduit 33. The anode exhaust stream is then provided by theanode recycle blower 123 into the mixer 105. The anode exhaust stream ishumidified in the mixer 105 by mixing with the steam provided from thesteam generator 103. The humidified anode exhaust stream is thenprovided from the mixer 105 via humidified anode exhaust stream conduit121 into the fuel inlet conduit 29 where it mixes with the fuel inletstream.

The air inlet stream is provided by a main air blower 125 from the airinlet conduit 33 into the anode cooler heat exchanger 100. The blower125 may comprise the single air flow controller for the entire system,as described above. In the anode cooler heat exchanger 100, the airinlet stream is heated by the anode exhaust stream via heat exchange.The heated air inlet stream is then provided into the air heat exchanger(cathode recuperator 200) via conduit 314 as shown in FIGS. 11F and 16.The heated air inlet stream is provided from heat exchanger 200 into thestack(s) 9 via the air inlet conduit and/or manifold 25.

The air passes through the stacks 9 into the cathode exhaust conduit 24and through conduit 24 and mixer 801 into the ATO 10. In the ATO 10, theair exhaust stream oxidizes the split first portion of the anode exhauststream from conduit 133 to generate an ATO exhaust stream. The ATOexhaust stream is exhausted through the ATO exhaust conduit 27 into theair heat exchanger 200. The ATO exhaust stream heats air inlet stream inthe air heat exchanger 200 via heat exchange. The ATO exhaust stream(which is still above room temperature) is then provided from the airheat exchanger 200 to the steam generator 103 via conduit 119. The heatfrom the ATO exhaust stream is used to convert the water into steam viaheat exchange in the steam generator 103, as shown in FIG. 11F. The ATOexhaust stream is then removed from the system via the exhaust conduit35. Thus, by controlling the air inlet blower output (i.e., power orspeed), the magnitude (i.e., volume, pressure, speed, etc.) of airintroduced into the system may be controlled. The cathode (air) andanode (fuel) exhaust streams are used as the respective ATO air and fuelinlet streams, thus eliminating the need for a separate ATO air and fuelinlet controllers/blowers. Furthermore, since the ATO exhaust stream isused to heat the air inlet stream, the control of the rate of single airinlet stream in conduit 33 by blower 125 can be used to control thetemperature of the stacks 9 and the ATO 10.

Thus, as described above, by varying the main air flow in conduit 33using a variable speed blower 125 and/or a control valve to maintain thestack 9 temperature and/or ATO 10 temperature. In this case, the mainair flow rate control via blower 125 or valve acts as a main systemtemperature controller. Furthermore, the ATO 10 temperature may becontrolled by varying the fuel utilization (e.g., ratio of currentgenerated by the stack(s) 9 to fuel inlet flow provided to the stack(s)9). Finally the anode recycle flow in conduits 31 and 117 may becontrolled by a variable speed anode recycle blower 123 and/or a controlvalve to control the split between the anode exhaust to the ATO 10 andanode exhaust for anode recycle into the mixer 105 and the fuel inletconduit 29.

FIG. 17A illustrates a side cross sectional view an anode recuperator1000 according to an embodiment. The anode recuperator 1000 may be ananode recuperator of a solid oxide fuel cell system. The anoderecuperator 1000 may include an annular fuel passage 1002 coupled to anannular pre-reformer 1004 and in fluid communication with the annularpre-reformer 1004. The annular pre-reformer 1004 may be configured toreceive an unreformed fuel stream from the first annular fuel passage1002. The annular pre-reformer 1004 may be an annular passage which maysupport a pre-reformer catalyst, such as nickel and/or rhodium, along alength of the annular pre-reformer 1004, and may reform the fuel streamreceived from the annular fuel passage 1002 to generate a reformed fuelstream. The annular pre-reformer 1004 may be coupled to a second annularfuel passage 1006 and in fluid communication with the second annularfuel passage 1006. The second annular fuel passage 1006 may beconfigured to receive the reformed fuel stream from the annularpre-reformer 1004. In an embodiment, the first annular fuel passage1002, annular pre-reformer 1004, and second annular fuel passage 1006may be one annular passage except that the annular pre-reformer portion1004 may contain catalyst. An annular anode exhaust passage 1008 maysurround at least a portion of the first annular fuel passage 1002, theannular pre-reformer 1004, and the second annular fuel passage 1006. Theannular anode exhaust passage 1008 may be thermally coupled to at leastthe first annular fuel passage 1002, the passage 1004 of thepre-reformer, and the second annular fuel passage 1006 such that ananode exhaust stream in the anode exhaust passage 1008 may provide heatto the unreformed fuel stream in the first annular fuel passage 1002,the passage 1004 of the pre-reformer, and the reformed fuel stream inthe second annular fuel passage 1006. In an embodiment, the catalyst orcatalyst support in the annular pre-reformer 1004 may be in directcontact with an inner wall of the anode exhaust passage 1008. Forexample, the catalyst or catalyst support in the annular pre-reformer1004 may rest against the anode exhaust passage 1008 and/or be brazed tothe anode exhaust passage 1008. In an embodiment, an inner wall of thefirst annular fuel passage 1002 may be configured to form a plenumsurrounded by the first annular fuel passage 1002, the second annularfuel passage 1006 may be configured to form a plenum surrounded by thesecond annular fuel passage 1006, and the annular pre-reformer 1004 maybe configured to form a plenum surrounded by the inner wall of theannular pre-reformer 1004. The three plenums may be in fluidcommunication, thereby forming a central plenum 1010 having an upperplenum portion surrounded by the first annular fuel passage 1002, apre-reformer plenum portion surrounded by the annular pre-reformer 1004,and a lower plenum portion surrounded by the second annular fuel passage1006.

FIG. 17B illustrates another embodiment of an anode recuperator 1000.Referring to FIG. 17B, the column 401 includes a catalyst housing 115disposed inside a central cavity of the anode recuperator 1000. Thecatalyst housing 115 includes one or more catalyst pucks 117A-117E, someor all of which constitute the above described pre-reformer 1004 in oneembodiment. Each puck 117 may include the same reformer catalyst, or oneor more of the pucks 117 may include different reformer catalysts.

FIG. 18A illustrates a perspective cross sectional view of the anoderecuperator 1000 described above with reference to FIG. 17A. In anembodiment, the annular pre-reformer 1004 may be a finned pre-reformer.In an embodiment, the first annular fuel passage 1002, the annularper-reformer 1004 and second annular fuel passage 1006 may be acontinuous annular passage containing a set of catalyst coated finsinserted in the middle which function as the pre-reformer 1004. Whileillustrated as a finned pre-reformer and finned fuel passages in variousfigures, the annular pre-reformer 1004 may have other configurationswhich may not incorporate fins. In another embodiment, the annularpre-reformer 1004 may be disposed in a gap between the first annularfuel passage 1002 and the second annular fuel passage 1006 if thepassages 1002 and 1006 do not form one continuous passage.

FIG. 18B illustrates a top cross sectional view of another embodiment ofthe anode recuperator 1000 shown in FIG. 17A. FIG. 18B illustrates thatthe annular anode exhaust passage 1008 surrounds (i.e., encircles) thefirst annular fuel passage 1002, the annular pre-reformer 1004 andsecond annular fuel passage 1006 (not visible in FIG. 18B). In anembodiment, the inner wall 1003 of the first annular fuel passage 1002may be configured to form an upper portion of the plenum 1010. In thismanner, the first annular fuel passage 1002 may surround (i.e.,encircle) the upper portion of the plenum 1010. While not visible inFIG. 18B, in a similar manner an inner wall of the second annular fuelpassage 1006 may be configured to form a lower portion of the plenum1010. The annular pre-reformer 1004 may be configured as a common wallhaving its inner surface coated with catalyst located between andseparating the first annular fuel passage 1002 and the annular anodeexhaust passage 1008. A common wall 1005 is located between the annularanode exhaust passage 1008 and the first annular fuel passage 1002, theannular pre-reformer 1004 and second annular fuel passage 1006.

FIG. 18C illustrates another top cross sectional view of anotherembodiment of the anode recuperator 1000 shown in FIG. 17A. Asillustrated in FIG. 18C, the common wall 1004 between the first annularfuel passage 1002 and the annular anode exhaust passage 1008 may be acatalyst coated corrugated fin with anode exhaust flowing vertically upthrough the annular anode exhaust passage 1008 and fuel flowingvertically down through the first annular fuel passage 1002. The anodeexhaust flowing in the annular anode exhaust passage 1008 may provideheat to the first annular fuel passage 1002, the annular pre-reformer1004, and/or the second annular fuel passage 1006, and any fuel flowstherein.

FIGS. 19A and 19B illustrate embodiments in which a portion of thecathode exhaust bypasses the ATO. FIG. 19A is a schematic diagramillustrating a hot box according to an embodiment while FIG. 19B is athree dimensional cut-away view of an ATO of the embodiment illustratedin FIG. 19A.

As illustrated in FIG. 19A an air inlet stream 1900 is provided from themanifold 25 through the fuel cell stack or column 9. The majority of thecathode exhaust 1901 (e.g. shown as element 1824 in FIG. 15C) isprovided from conduit 24 through mixer 801 into the ATO 10. In a firstembodiment, one or more holes or slits 1902 are provided in a lowerportion of the outer cylinder 10A of ATO 10. A first cathode exhaustbypass stream 1906 flows from conduit 24 through the hole or slit 1902in the outer cylinder 10A of ATO 10, thereby bypassing the mixer 801 andthe catalyst 10C in the ATO. The first cathode exhaust bypass stream1906 is then mixed with the ATO exhaust stream 1909 and provided intothe cathode recuperator 200 via conduit 27. In a second embodiment, oneor more holes or slits 1904 are provided in the ATO skirt 1603. A secondcathode exhaust bypass stream 1908 flows from conduit 24 through thehole or slit 1904 in the ATO skirt 1603, entering the conduit 27 outsidethe anode hub structure 600. The second cathode exhaust bypass stream1908 is then mixed with the ATO exhaust stream 1909 and provided intothe cathode recuperator 200 via conduit 27. In another embodiment, holesor slits 1902, 1904 are provided in a lower portion of the outercylinder 10A of ATO 10 and in the ATO skirt 1603. In this embodiment,two cathode exhaust bypass streams 1906, 1908 are provided thoughconduit 27 into the cathode recuperator 200.

Without wishing to be bound by a particular theory, it is believed thatif a fuel cell system, such as the system described above, has a highfuel utilization (e.g., greater than 85% fuel utilization, such as 87%to 92% fuel utilization), there will be less fuel in the fuel exhauststream. The lower amount of fuel in the fuel exhaust stream may resultin a lower ATO temperature, which may lead to an undesirable amount ofcarbon monoxide present in the ATO stream if carbon monoxide is notconverted to carbon dioxide in the ATO. While fresh fuel can be added tothe ATO to increase the ATO temperature, this would decrease the systemfuel utilization. Thus, in order maintain a high fuel utilization in thefuel cell system, less air is provided to the ATO 10 to maintain ahigher ATO temperature, such as a temperature of at least 875° C., suchas 890° C. to 925° C. This avoids the necessity to use additional freshfuel in the ATO 10 to maintain a sufficiently high ATO temperature.Since additional fuel is not provided to the ATO, the system fuelutilization is maintained without providing an undesirable amount ofcarbon monoxide into the ATO exhaust stream.

FIG. 20 is a schematic process flow diagram illustrating a hot boxutilizing the embodiment illustrated in FIGS. 19A and 19B. Asillustrated in FIG. 20, air from the air heat exchanger 200 is providedto the fuel cell stack or column 9. The majority of the cathode exhaust1901 is provided to the ATO 10 via an air exhaust conduit or manifold24. In an embodiment, a first cathode exhaust bypass stream 1906 exitsthe fuel cell stack or column 9 and is provided to a lower portion ofthe ATO 10. The first cathode exhaust bypass stream 1906 joins themajority of the cathode exhaust 1901 that has been processed by the ATO,exiting the ATO via conduit 27. In another embodiment, the secondcathode exhaust bypass stream 1908 exits the fuel cell stack or column9, passes through the ATO skirt 1603 and is provided to conduit 27 andcombined with the majority of the cathode exhaust 1901 from the ATO 10.Without wishing to be bound by a particular theory, it is believed thatthe process flow illustrated in FIG. 20 may result in greater than 95%,such as 99-100% CO conversion to CO₂ in the ATO 10.

FIGS. 21A and 21B illustrate an embodiment of a bimetal louver 2100 thatmay be used to close and open the at least one hole or slit 1902 inouter cylinder 10A of ATO 10 or the at least one hole or slit 1904 inATO skirt to adjust amount of cathode exhaust that bypasses the ATO 10as a first and/or second cathode exhaust bypass stream 1906 and/or 1908as a function of ATO temperature. As illustrated in FIG. 21A, thebimetal louver 2100 includes a first metal sheet, strip or plate 2102with a first coefficient of thermal expansion and a second metal, sheet,strip or plate 2104 with a second coefficient of thermal expansiondifferent from the first coefficient of thermal expansion. The two metalsheets, strips or plates 2102, 2104 are joined together at the ends withconnectors 2106. At low temperature, the first metal sheet, strip orplate 2102 and the second metal sheet, strip or plate 2104 havesubstantially the same length and lay flat against each other. As thetemperature increases, the metal with the higher coefficient of thermalexpansion expands more rapidly than the metal with the lower coefficientof thermal expansion. Because the two metal sheets, strips or plates2102, 2104 are bound to each other at or near the ends of the metalsheets, strips or plates 2102, 2104, the metal sheet, strip or plate2102, 2104 with the higher coefficient of thermal expansion cannotexpand past the metal sheet, strip or plate 2102, 2104 with the lowercoefficient of thermal expansion. Rather, as illustrated in FIG. 21A, abending force is generated which causes the bimetal louver 2100 to bendand close at least one hole or slit 1902, 1904, thereby decreasing theamount of cathode exhaust which can bypass the catalyst section of theATO 10. As the ATO temperature cools due to the increased amount ofcathode exhaust provided through the catalyst section of the ATO, thebimetal louver unbends and open the hole or slit 1902, 1904, increasingthe amount of cathode exhaust flowing through the hole or slit 1902,1904 and bypassing the catalyst section of the ATO 10, as shown in FIG.21B. This again increases the ATO temperature. The bending and unbendingprocess automatically repeats as the temperature of the ATO increasesand decreases to maintain the ATO temperature in a desired range (e.g.,between 890° C. and 925° C.).

An embodiment is drawn to a method of operating a fuel cell systems,such as a solid oxide fuel cell (SOFC) system, including splitting acathode exhaust from one or more fuel cell stacks in the system into amajority cathode exhaust stream comprising greater than 50% of thecathode exhaust and a first cathode exhaust bypass stream, providing themajority cathode exhaust stream to an inlet of an anode tail gasoxidizer (ATO) containing a catalyst, and providing the first cathodeexhaust bypass stream downstream of the catalyst such that the bypassstream bypasses the catalyst.

In some embodiments, providing the first cathode exhaust bypass streamdownstream of the catalyst comprises at least one of: directing thefirst cathode exhaust bypass stream through a hole or slit in a lowerportion of the ATO proximal to an outlet of the ATO or directing thefirst cathode exhaust bypass stream though a hole or slit in an ATOskirt. As used herein, an ATO skirt is a horizontal surface or platethat surrounds the bottom of the ATO.

In an embodiment, the first cathode exhaust bypass stream is directedthrough the hole or slit in a lower portion of the ATO proximal to theoutlet of the ATO. In another embodiment, the first cathode exhaustbypass stream is directed though the hole or slit in the ATO skirt.Another embodiment further includes controlling a magnitude (e.g. flowrate) of the first cathode exhaust bypass stream with a bimetal louver.Another embodiment includes directing the first cathode exhaust bypassstream through the hole or slit in a lower portion of the ATO proximalto an outlet of the ATO, and directing the second cathode exhaust bypassstream though the hole or slit in an ATO skirt. Another embodimentfurther includes controlling a relative magnitude (e.g. flow rate) ofthe first and/or second cathode exhaust bypass streams with bimetallouvers positioned adjacent to the respective holes or slits.

An embodiment is drawn to a solid oxide fuel cell (SOFC) systemincluding at least one fuel cell stack, an anode tail gas oxidizer(ATO), an ATO skirt and at least one hole or slit in a lower portion ofthe ATO or in the ATO skirt. One embodiment includes the at least onehole and slit in the lower portion of the ATO. Another embodimentincludes the at least one hole and slit in the ATO skirt. Anotherembodiment includes a bimetal louver covering the at least one hole orslit in a lower portion of the ATO or the at least one hole or slit inthe ATO skirt. Another embodiment includes a bimetal louver covering theat least one hole or slit in a lower portion of the ATO and a bimetallouver covering the at least one hole or slit in the ATO skirt.

FIGS. 22A-22E illustrate embodiments in which additional cooling isprovided to the cathode recuperator 200. FIG. 22A is a schematiccross-section illustrating cathode flows in a SOFC system withoutadditional cooling to the cathode recuperator 200 while FIG. 22B is aschematic cross-section illustrating cathode flows in a SOFC system withadditional cooling to the cathode recuperator 200. In the cathoderecuperator 200, the heat available in the cathode exhaust streamexceeds the heat required by the incoming air. This is due to the lowerthan desired overall heat rejection from the hot box through the hot boxinsulation 1950 (e.g., free flow insulation). This is also due to thefact that the mass flow of the ATO exhaust stream is higher than themass flow of the air inlet stream because of the blending of theoxidation products from the fuel exhaust stream into the cathode exhauststream.

As illustrated in FIG. 22A, air is provided to the SOFC system via themain air inlet 50. The incoming air is provided to the cathoderecuperator 200 in which it is heated by the ATO 10 exhaust stream. Theheated air is then provided to the fuel cell stack or column 9 throughthe manifold 25. From the stack or column 9, cathode exhaust is providedto the ATO 10 in which un-reacted fuel is oxidized with the cathodeexhaust. The ATO exhaust is then passed through a carbon monoxidescrubber 2201 before exiting the SOFC system via the exhaust outlet 52.

FIG. 22B illustrates an embodiment in which additional cooling air isprovided to cool the cathode recuperator 200. In this embodiment, anadditional air supply 2210, such as a fan or blower, is provided whichsupplies additional cool air separate from the air provided from themain air inlet 50. The fan or blower 2210 is located outside the hot box1 insulation 1950 but inside system cabinet. In an embodiment, a heatexchanger 2200 is provided on the outside of and in heat exchangerelationship with the cathode recuperator 200 but inside the hot box 1insulation 1950. In an embodiment, the heat exchanger 2200 includes aninner concentric conduit 2202 a and an outer concentric conduit 2202 b.As illustrated in the plan view of FIG. 22C, the outer wall 202 c of thecathode recuperator 200 forms the inner wall of the inner concentricconduit 2202 a. A first concentric ring 2203 forms the outer wall of theinner concentric conduit 2202 a and the inner wall of the outerconcentric conduit 2202 b. A second concentric ring 2205 forms the outerwall of the outer concentric conduit 2202 b.

In one embodiment, cooling air is provided through an air inlet conduit2204 which extends through the hot box 1 insulation 1950 to the fan orblower 2210 and then to the inner concentric conduit 2202 a of the heatexchanger 2200. The cooling air removes some heat from the air inletstream passing through the cathode recuperator 200. The cooling air isthen passed from the inner concentric conduit 2202 a to the outerconcentric conduit 2202 b and exits the hot box 1 via an air outletconduit 2212 from the outer concentric conduit 2202 b. In an alternativeembodiment, the location of the air inlet conduit 2204 and the airoutlet 2212 conduit are reversed. That is, the cooling air is providedto the outer concentric conduit 2202 b and flowed to the innerconcentric ring 2202 a before exiting the heat exchanger 2200. Inanother embodiment, the inner concentric conduit 2202 a may replacedwith a corrugated fin assembly (e.g., having inlet and outlet airstreams flowing on opposite side of the fin similar to the configurationshown in FIG. 18C) and the outer concentric conduit 2202 b is not used.In another embodiment, the inner concentric conduit 2202 a may replacedwith a corrugated fin assembly which is surrounded by second concentricring 2205. In another embodiment, fins are brazed to outer wall of thecathode recuperator 202 c and extend into the inner concentric conduit2202 a for improved heat transfer.

In an embodiment, the length of the heat exchanger 2200 is shorter thanthe length of the cathode recuperator 200. The short length may be useto reduce the amount of heat transfer from the cathode recuperator outershell 200 c or limit the outlet temperature of the outlet air from theheat exchanger 2200 or to eliminate the transfer of higher grade heatfrom the cathode recuperator 200. The shorter length may also reduce oreliminate any potential chromium evaporation from the hottest end of theouter shell of the heat exchanger 2200. In an alternative embodiment,the heat exchanger 2200 is the same length as the cathode recuperator200. In this embodiment, heat transfer is maximized from the heatexchanger without explicitly taking heat from the high temperature ATOexhaust. In another alternative embodiment, the heat exchanger 2200 islonger than the length of the cathode recuperator 200 to removeadditional heat from the high temperature ATO exhaust.

FIG. 22D is a plan view illustrating the SOFC system of FIG. 22Baccording to another embodiment. In this embodiment, an insulation layer2207 is provided between the inner concentric conduit 2202 a and theouter concentric 2202 b. The insulation layer 2207 may be locatedbetween the outer wall of the cathode recuperator 202 c and the firstconcentric ring 2203 of heat exchanger 2200 or between the firstconcentric ring 2203 of heat exchanger 2200 and the second concentricring 2205 of the heat exchanger 2200. In this embodiment, the insulationdecreases reverse heat transfer from the outer concentric conduit 2202 bto the inner concentric conduit 2202 a.

FIG. 22E is a plan view illustrating the SOFC system of FIG. 22Baccording to another embodiment. In this embodiment, the firstconcentric ring 2203 of heat exchanger 2200 includes perforations 2209(e.g. slits, holes or another suitable openings). In this embodiment,the perforations 2209 allow mixture of air between the inner concentricconduit 2202 a and the outer concentric 2202 b, thereby limiting themaximum temperature of the exit air from the heat exchanger 2200.

In another embodiment, the location of the cathode recuperator 200inside the hot box is lowered relative to the embodiment illustrated inFIGS. 22A and 22B. This results in the introduction of the air inletstream lower in the hot box. The lower position results in additionallow grade heat transfer surface available for heating the additionalcooling air, without necessarily using the high grade heat in the ATOexhaust for heating the additional cooling air. Rather, the high gradeheat would be retained for heating the incoming air inlet stream in thecathode recuperator 200.

Reducing the exhaust outlet temperature from the cathode recuperator 200may affect the chromium oxide scrubber 2201. For example, a lower volumeair flow results in a lower superficial velocity which results in alower pressure drop across the chromium oxide scrubber 2201. Inaddition, increased retention time may result in better capture ofchromium oxide, especially with at a lower temperature.

The air exit stream of the heat exchanger 2200 could be used forcombined heat and power. For example, because the additional air is onlyair (no extra CO₂ or water), it could be added directly to an air streamused for building heating.

While the heat exchanger 2200 is described in the above embodiments asusing cooling air as the cooling medium to cool the cathode recuperator,other cooling media and methods may be used instead. In one embodiment,any cooler gas stream may be provided to the heat exchanger, for examplesuch as a gas stream that can be vented to the atmosphere. Such gasincludes nitrogen or carbon dioxide from liquid nitrogen or carbondioxide containing cylinders, or any other precooled gas, e.g. cooledvent gases from an adjacent air separation plant. In another embodiment,the heat exchanger 2200 can be a refrigerator conduit loop configured toprovide indirect cooling of the cathode exhaust stream in the cathoderecuperator by refrigeration. Thus, a coolant may flow through the heatexchanger 2200 to and from a refrigerator. In another embodiment,cooling water may be passed through the heat exchanger 2200 and/orprovided from the heat exchanger into the cathode exhaust stream, e.g.,by mixing and/or evaporation.

An embodiment is drawn to a method of operating a fuel cell systemincluding providing an air inlet stream to the SOFC system via a mainair inlet, providing the air inlet stream from the main air inlet to acathode recuperator and providing a cooling medium, such as cooling air,to a heat exchanger to cool the cathode recuperator. In variousembodiments, the cooling medium comprises at least one of air, water,refrigerant coolant, nitrogen or carbon dioxide. In an embodiment, theheat exchanger is affixed to an outer wall of the cathode recuperator.In an embodiment, providing cooling air comprises providing the coolingair to an inner conduit surrounding the cathode recuperator and removingthe cooling air via an outer conduit surrounding the inner conduit. Inan embodiment, providing cooling air comprises providing the cooling airto an outer conduit surrounding an inner conduit surrounding the cathoderecuperator and removing the cooling air via the inner conduit. In anembodiment, the heat exchanger has a length smaller than a length of thecathode recuperator. In an embodiment, the heat exchanger has a lengthequal to a length of the cathode recuperator. In an embodiment, the heatexchanger has a length greater than a length of the cathode recuperator.In an embodiment, insulation is located between inner and outer conduitsof the heat exchanger. In another embodiment, the cooling air flowsthrough perforations between inner and outer conduits of the heatexchanger. In an embodiment, the method further comprises removing thecooling air from the heat exchanger and providing cooling air forbuilding temperature control.

An embodiment is drawn to a fuel cell system, such as a solid oxide fuelcell system comprising a cathode recuperator, a heat exchanger thermallycoupled to the cathode recuperator and an air supply configured toprovide cooling air to the heat exchanger. In an embodiment, the heatexchanger comprises an inner conduit surrounding the cathode recuperatorand an outer conduit surrounding the inner conduit. In an embodiment,the air supply is configured to provide air to the inner conduit. In anembodiment, the air supply is configured to provide air to the outerconduit. In an embodiment, the system further includes a layer ofinsulation located between the inner and the outer conduits. In anotherembodiment, the system further includes perforations in a common walllocated between the inner and the outer conduits. In an embodiment, aninlet of the heat exchanger is connected to at least one of a coolingair fan or blower, a refrigerator, a liquid nitrogen vessel, a liquidcarbon dioxide vessel, cooled vent gases conduit from an air separationplant, or a water source.

Advantages of the embodiments illustrated in FIGS. 22B-22E include:reduced parasitic power relative to increasing the air inlet streamflow, reduced cathode exhaust exit temperature relative to holding theair inlet stream constant, reduced amount of free flow insulation 1950required per hot box 1 due to active cooling and allowing the cathoderecuperator 200 to be pushed further outward while maintaining the sameenclosure size.

Any one or more features of any embodiment may be used in anycombination with any one or more other features of one or more otherembodiments. The construction and arrangements of the fuel cell system,as shown in the various exemplary embodiments, are illustrative only.Although only a few embodiments have been described in detail in thisdisclosure, many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter described herein. Someelements shown as integrally formed may be constructed of multiple partsor elements, the position of elements may be reversed or otherwisevaried, and the nature or number of discrete elements or positions maybe altered or varied. The order or sequence of any process, logicalalgorithm, or method steps may be varied or re-sequenced according toalternative embodiments. Other substitutions, modifications, changes andomissions may also be made in the design, operating conditions andarrangement of the various exemplary embodiments without departing fromthe scope of the present disclosure.

The invention claimed is:
 1. A fuel cell system, comprising: at leastone fuel cell stack; an anode tail gas oxidizer (ATO) containing acatalyst; an ATO skirt extending from a lower portion of the ATO; afirst hole or slit formed in the lower portion of the ATO, downstream ofthe catalyst; and a cathode exhaust conduit configured to receivecathode exhaust from the at least one fuel cell stack and provide: amajor exhaust stream to an upper portion of the ATO, such that the majorexhaust stream flows sequentially through the upper portion of the ATO,the catalyst, and the lower portion of the ATO, before exiting the ATO;and a first bypass exhaust stream to the first hole or slit, such thatthe first bypass exhaust stream bypasses the catalyst and flows throughthe bottom portion of the ATO before exiting the ATO.
 2. The system ofclaim 1, further comprising a first bimetal louver configured to controla flow rate of the first bypass exhaust stream through the first hole orslit based on a temperature of the ATO.
 3. The system of claim 2,wherein the the first bimetal louver comprises: a first metal sheethaving a first coefficient of thermal expansion; and a second metalsheet disposed on the first metal sheet and having a second coefficientof thermal expansion different from the first coefficient of thermalexpansion.
 4. The system of claim 2, wherein the first bimetal louver isconfigured such that the flow rate of the first bypass exhaust stream isrelatively high, when the temperature of the ATO is relatively low, andthe flow rate of the first bypass exhaust stream is relatively low, whenthe temperature of the ATO is relatively high.
 5. The system of claim 1,further comprising: a second hole or slit formed in the ATO skirt andconfigured to receive a second bypass exhaust stream from the cathodeexhaust conduit; and a second bimetal louver configured to control aflow rate of the second bypass exhaust stream through the second hole orslit based on a temperature of the ATO.
 6. The system of claim 1,wherein the major exhaust stream comprises a majority of the cathodeexhaust received by the cathode exhaust conduit.
 7. A fuel cell system,comprising: at least one fuel cell stack; an anode tail gas oxidizer(ATO) containing a catalyst; an ATO skirt extending from a lower portionof the ATO; a first hole or slit formed in the ATO skirt; a cathodeexhaust conduit configured to receive cathode exhaust from the at leastone fuel cell stack and provide: a major exhaust stream to an upperportion of the ATO, such that the major exhaust stream flows through theupper portion of the ATO, the catalyst, and the lower portion of theATO, before exiting the ATO as an ATO exhaust stream; and a first bypassexhaust stream to the first hole or slit, such that the first bypassexhaust stream bypasses the ATO and mixes with the ATO exhaust stream.8. The fuel cell system of claim 7, further comprising a first bimetallouver configured to control a flow rate of the first bypass exhauststream through the first hole or slit based on a temperature of the ATO.9. The fuel cell system of claim 8, wherein the first bimetal louvercomprises: a first metal sheet having a first coefficient of thermalexpansion; and a second metal sheet disposed on the first metal sheetand having a second coefficient of thermal expansion different from thefirst coefficient of thermal expansion.
 10. The fuel cell system ofclaim 8, wherein the first bimetal louver is configured such that theflow rate of the first bypass exhaust stream is relatively high, whenthe temperature of the ATO is relatively low, and the flow rate of thefirst bypass exhaust stream is relatively low, when the temperature ofthe ATO is relatively high.
 11. The system of claim 7, furthercomprising: a second hole or slit formed in the lower portion of the ATOand configured to receive a second bypass exhaust stream from thecathode exhaust conduit; and a second bimetal louver configured tocontrol a flow rate of the second bypass exhaust stream through thesecond hole or slit based on a temperature of the ATO.
 12. The system ofclaim 7, wherein the major exhaust stream comprises a majority of thecathode exhaust received by the cathode exhaust conduit.